The IMS is an architecture for enabling the provision of IP based multimedia services in 3G and subsequent mobile and converged fixed/mobile communications networks. Features of IMS are being standardized under the 3rd Generation Partnership Programme (3GPP) and 3GPP2. Various network elements in the IMS architecture will now be described by way of background information.
The Home Subscriber Server (HSS) is a database that stores subscriber data, performs authentication and authorization of subscribers, and can provide the location of a subscriber user terminal to enable mobility. In the description of IMS used by Verizon Wireless™ the HSS is called a Subscriber Data Manager (SDM) and the terms HSS and SDM are used interchangeably in this document.
Call Session Control Functions (CSCFs) are used to manage the establishment and disestablishment of calls or sessions in the network in accordance with the instructions of subscribers and ASs using the Session Initiation Protocol (SIP). In the home network of a subscriber, a network element called the Serving CSCFs (S-CSCF) provides the call and session handling functions for the subscriber. The S-CSCF does not maintain subscriber data locally but queries the HSS for such data using the Cx interface based on the DIAMETER protocol. The interrogating-CSCF (I-CSCF) is the contact point within an operator's network for all connections destined to a subscriber of that network operator, or for a roaming subscriber currently located within that network operator's service area. The Proxy CSCF (P-CSCF) is often the entry into the signaling network, The P-CSCF helps setup and manage sessions, and it forwards messages between IMS networks. It is the first point of contact for a client accessing the IMS network, whether that client is in its home network or roaming in a visited network.
Application Servers (ASs) are used to manage the provision of advanced services to subscribers. ASs may be provided by the network operator and resident in a home network of a subscriber or they may be provided by third parties and in a foreign network. ASs execute applications which provide the control logic for the provision of advanced services such as messaging, conference calling, voice mail, presence services, location based services and so on. Other elements such as media servers may be used to provide some of the required services themselves. ASs may also need to query the HSS for subscriber data and use the Sh interface (which is also based on DIAMETER) to do so.
In the description of IMS used by Verizon Wireless™, there are various network elements known as the Application Manager (AM), Bearer Manager (BM) and Security manager (SM) each of which may query the SDM for subscriber data as well as non-IMS applications. In general, the AMs correspond in 3GPP and 3GPP2 terminology to both CSCFs and ASs.
In some networks, particularly networks with many subscribers or geographically wide spread networks, multiple HSSs are implemented at different physical sites. This can improve capacity, reduce latency and provide redundancy. In one approach (replication), identical subscriber data is stored at all HSS sites and any HSS can service a request in respect of any subscriber. A typical requirement is for dual or triple site deployment. A network element can interact with any site but typically uses nearest for lower latency. In another approach (partitioning), each HSS site is responsible for a subset of the entire subscriber data for the network. Redundancy may still be employed, but deployment is partition specific. A network element must query an HSS site (partition) responsible for the associated subscriber. Accordingly, a routing function is needed to map the subscriber data requests of network elements to the correct FISS. In 3GPP and 3GPP2 IMS, this routing function is called the Subscriber Location Function (SLF). The SLF and HSS communicate using the DIAMETER protocol.
FIG. 1 shows an exemplary implementation of a partitioned SDM (ie HSS) in a large communications network. The network is divided into a number (in this case 5) geographic regions 10 (only two shown) each having an SDM 14—ie SDMs A, B, C, D and E—forming a partitioned SDM service 12. Each SDM acts as home SDM for subscriber's resident in it's geographic region. Multimedia and data (MMD) services are provided by AM, BM and other functional elements in each geographic region's cluster 16. A subscriber will always register with their home SDM. The assumption is that vast bulk of subscriber activity will be in home region and so interaction will be with home region elements (AM, BM etc). However, network elements in any region will need to access the subscriber's SDM to terminate sessions to that subscriber and also to support subscriber registration due to roaming between regions, for example. Elements will need to have requests routed to the correct (home) SDM and so an SLF is used.
According to present IMS standards, the SLF receives SDM queries and redirects the querying network element to the appropriate SDM. For example, 3GPP (23.228, 29.228 and 29.329) identifies an SLF which operates as a Cx and Sh re-direct routing service. As shown in FIG. 2 at step 1, a network element AM 20 sends the intended Cx/Sh query to SLF 22. The SLF uses an internal routing table to return a Diameter redirect reply providing the address of the appropriate SDM 24—in this example SDM B—to handle the Cx query (step 2). At step 3, the AM resends the Cx/Sh query to SDM B. SDM B then returns the Cx response to the AM (step 4). Note that the SLF is a distinct Diameter service requiring each AM to maintain separate SLF and SDM sessions. Typically, AMs make their first query to the SLF then cache the returned SDM address for use in subsequent queries.
The SLF approach used in present IMS standards suffers from certain drawbacks.
Network complexity can be a problem since the SLF must appear as a distinct service to the SDM and each network element must maintain separate Diameter sessions to the SLF and the SDM
Handling a change of the home SDM for a subscriber (as a result of a site failure through disaster or a matter of administrative reorganization) can also be a problem. This requires that each network element that might interact with the SDM on behalf of the subscriber be updated with new Diameter connection to the new SDM. This is a significant processing overhead for each subscriber that is “moved” from one SDM to another and for large subscriber moves a very high processing overhead with impact on latency as well.
Also, using the re-direct routing service forces network elements to be capable of handling Diameter re-direct replies which adds complexity to the network elements. Moreover, increased processing overhead is incurred at the network element as a result of redirection.
An object of the present invention is to provide an improved SLF approach.